U.S. patent number 4,054,835 [Application Number 05/744,066] was granted by the patent office on 1977-10-18 for rapid response generating voltmeter.
This patent grant is currently assigned to General Electric Company. Invention is credited to David Russell Humphreys, Edward Joseph Los.
United States Patent |
4,054,835 |
Los , et al. |
October 18, 1977 |
Rapid response generating voltmeter
Abstract
A rapid-response generating voltmeter for measuring a high d-c
voltage between a high voltage electrode and ground comprises two
sets of stator segments, the segments of each set being
electrically interconnected and each set being connected through
its own capacitor to ground. Each set of stator segments has a
time-varying capacitance with respect to the high voltage
electrode, but the sum of these time-varying capacitances is
maintained substantially constant with time. First and second
circuit means respectively sense the voltages e.sub.1 and e.sub.2
present across said two capacitors and develop first and second
intermediate signals e.sub.5 and e.sub.6 substantially proportional
to e.sub.1 and e.sub.2, respectively, minus, in each case, an error
voltage attributable to discharge of the associated one of said
capacitors. Summing means develops an output signal substantially
proportional to the instantaneous sum of e.sub.5 and e.sub.6, and
this output signal is substantially proportional to the high d-c
voltage being measured.
Inventors: |
Los; Edward Joseph (Pittsfield,
MA), Humphreys; David Russell (Dalton, MA) |
Assignee: |
General Electric Company
(Philadelphia, PA)
|
Family
ID: |
24991292 |
Appl.
No.: |
05/744,066 |
Filed: |
November 22, 1976 |
Current U.S.
Class: |
324/109; 324/130;
324/72 |
Current CPC
Class: |
G01R
15/16 (20130101); G01R 29/12 (20130101) |
Current International
Class: |
G01R
15/16 (20060101); G01R 15/14 (20060101); G01R
29/12 (20060101); G01R 005/28 (); G01R
029/08 () |
Field of
Search: |
;324/109,32,72,130
;330/9 ;324/120 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Trump et al; "Generating Voltmeter . . . "; Rev. of Sci.
Instruments; Feb. 1940; pp. 54-56..
|
Primary Examiner: Rolinec; Rudolph V.
Assistant Examiner: Karlsen; Ernest F.
Attorney, Agent or Firm: Freedman; William
Claims
What we claim as new and desire to secure by Letters Patent of the
United States is:
1. In a rapid-response generating voltmeter for measuring a high
d-c voltage applied between first and second spaced-apart
points,
a. a first electrode adapted to operate at the potential of said
first point,
b. a second electrode spaced from said first electrode and
comprising a first stator including a plurality of segments
arranged in angularly-spaced relationship about a predetermined
center and electrically connected together so as to be at the same
potential as each other,
c. a first capacitor connected between said first stator and said
second point,
d. a third electrode spaced from said first electrode, electrically
insulated from said second electrode, and comprising a second
stator including a plurality of segments arranged in
angularly-spaced relationship about said center and positioned
physically between the segments of said first stator, the segments
of said second stator being electrically interconnected so as to be
at the same potential as each other,
e. a second capacitor connected between said second stator and said
second point,
f. capacitance-varying means for periodically varying substantially
simultaneously in opposite senses the capacitance between said
first electrode and said second electrode and the capacitance
between said first electrode and said third electrode, said
capacitance-varying means comprising a rotatable rotor having vanes
normally at the potential of said second point located between said
first electrode on one hand and the segments of said stators on the
other hand, rotation of said rotor causing said vanes to gradually
cover the segments of one stator with respect to said first
electrode as said vanes gradually uncover the segments of the other
stator with respect to said first electrode,
g. said rotor vanes effectively completely covering the segments of
said first stator at successive instants spaced in time by
predetermined successive first intervals,
h. first circuit means for sensing the voltage across said first
capacitor and for developing during said first intervals a first
intermediate signal varying in magnitude directly in accordance
with: the voltage then present across said first capacitor minus a
first error voltage substantially equal to the voltage across said
first capacitor at the start of each of said first intervals,
i. said rotor vanes effectively completely covering the segments of
said second stator at successive instants spaced in time by
predetermined successive second intervals,
j. second circuit means for sensing the voltage across said second
capacitor and for developing during said second intervals a second
intermediate signal varying in magnitude directly in accordance
with the voltage then present across said second capacitor minus an
error voltage substantially equal to the voltage across said second
capacitor at the start of each of said second intervals,
k. and summing means for developing an output signal substantially
proportional to the instantaneous sum of said first and second
intermediate signals.
2. In a rapid-response generating voltmeter for measuring a high
d-c voltage applied between first and second spaced-apart
points,
a. a first electrode adapted to operate at the potential of said
first point,
b. a second electrode spaced from said first electrode and
comprising a first stator including a plurality of segments
arranged in angularly-spaced relationship about a predetermined
center and electrically connected together so as to be at the same
potential as each other,
c. a first capacitor connected between said first stator and said
second point,
d. a third electrode spaced from said first electrode, electrically
insulated from said second electrode, and comprising a second
stator including a plurality of segments arranged in
angularly-spaced relationship about said center and positioned
physically between the segments of said first stator, the segments
of said second stator being electrically interconnected so as to be
at the same potential as each other,
e. a second capacitor connected between said second stator and said
second point,
f. capacitance-varying means for periodically varying substantially
simultaneously in opposite senses the capacitance between said
first electrode and said second electrode and the capacitance
between said first electrode and said third electrode, said
capacitance-varying means comprising a rotatable rotor having vanes
normally at the potential of said second point located between said
first electrode on one hand and the segments of said stators on the
other hand, rotation of said rotor causing said vanes to gradually
cover the segments of one stator with respect to said first
electrode as said vanes gradually uncover the segments of the other
stator with respect to said first electrode,
g. first circuit means for sensing the voltage across said first
capacitor while said rotor is rotating and for developing a first
intermediate signal varying in magnitude directly in accordance
with: the instantaneous voltage then present across said first
capacitor minus a first error voltage substantially equal to the
voltage appearing across said first capacitor at the
immediately-preceding instant when the segments of said first
stator were effectively completely covered by the vanes of said
rotating rotor,
h. second circuit means for sensing the voltage across said second
capacitor while said rotor is rotating and for developing a second
intermediate signal varying in magnitude directly in accordance
with: the instantaneous voltage then present across said second
capacitor minus a second error voltage substantially equal to the
voltage appearing across said second capacitor at the
immediately-preceding instant when the segments of said second
stator were effectively completely covered by the vanes of said
rotating rotor, and
i. summing means for developing an output signal substantially
proportional to the instantaneous sum of said first and second
intermediate signals.
3. The voltmeter of claim 2 in which at least one of said first and
second circuit means includes gain adjusting means adjustable to
make the intermediate signal developed by the associated circuit
means substantially equal in amplitude to that of the other
intermediate signal when the voltmeter is measuring a constant d-c
high voltage.
4. The voltmeter of claim 2 in which said first circuit means
includes:
a. means for deriving a first signal substantially proportional by
a predetermined multiplier to the instantaneous voltage across said
first capacitor,
b. a first sample-and-hold circuit that acts to provide a first
correction signal proportional also by said predetermined
multiplier to the voltage across said first capacitor each time the
segments of said first stator are effectively completely covered by
the vanes of said rotor, the amplitude of said correction signal
being maintained substantially constant between successive instants
at which said segments of said first stator are effectively
completely covered,
c. and first difference circuit means for subtracting said first
correction signal from said first signal and for developing said
first intermediate signal substantially in proportion to the
difference resulting from said substraction.
5. The voltmeter of claim 4 in which said second circuit means
includes:
a. means for deriving a second signal substantially proportional by
a predetermined second multiplier to the instantaneous voltage
across said second capacitor,
b. a second sample-and-hold circuit that acts to provide a second
correction signal proportional also by said second predetermined
multiplier to the voltage across said second capacitor each time
the segments of said second stator are effectively completely
covered by the vanes of said rotor, the amplitude of said second
correction signal being maintained substantially constant between
successive instants at which the segments of said second stator are
effectively completely covered,
c. and second different circuit means for subtracting said second
correction signal from said second signal and for developing said
second intermediate signal substantially in proportion to the
difference resulting from said latter subtraction.
6. The generating voltmeter of claim 2 in which:
a. said first intermediate signal developed by said first circuit
means is a time-varying signal having a dominant frequency
proportional to the rotational speed of said rotor,
b. said second intermediate signal developed by said second circuit
means is a time-varying signal having a dominant frequency
proportional to the rotational speed of said rotor,
c. said two intermediate signals are approximately 180.degree. out
of phase with each other, considered with respect to their dominant
frequency waveforms.
7. The generating voltmeter of claim 6 in which the sum of (i) the
capacitance between said first electrode and said second electrode
and (ii) the capacitance between said first electrode and said
third electrode remains substantially constant during rotation of
said rotor.
8. The generating voltmeter of claim 6 in which: said first and
second intermediate signals sum to a constant when the voltmeter is
measuring a constant level of high voltage.
9. The generating voltmeter of claim 8 in which the sum of (i) the
capacitance between said first electrode and said second electrode
and (ii) the capacitance between said first electrode and said
third electrode remains substantially constant during rotation of
said rotor.
10. The generating voltmeter of claim 2 in which the sum of: (i)
the capacitance between said first electrode and said second
electrode and (ii) the capacitance between said first electrode and
said third electrode remains substantially constant during rotation
of said rotor.
11. The generating voltmeter of claim 4 in which the sum of: (i)
the capacitance between said first electrode and said second
electrode and (ii) the capacitance between said first electrode and
said third electrode remains substantially constant during rotation
of said rotor.
12. The generating voltmeter of claim 5 in which the sum of: (i)
the capacitance between said first electrode and said second
electrode and (ii) the capacitance between said first electrode and
said third electrode remains substantially constant during rotation
of said rotor.
13. The generating voltmeter of claim 2 in which:
a. said first capacitor is large with respect to the capacitance
between said first electrode and said second electrode even when
the stator segments of said second electrode are effectively
completely uncovered, and
b. said second capacitor is large with respect to the capacitance
between said first electrode and said third electrode even when the
stator segments of said third electrode are effectively completely
uncovered.
14. The generating voltmeter of claim 2 in which said first and
second intermediate signals sum to a constant when the voltmeter is
measuring a constant level of high voltage.
Description
BACKGROUND
This invention relates to a generating voltmeter and, more
particularly, to a generating voltmeter that has a rapid response
to changes in the voltage being measured.
A generating voltmeter is a device which uses a time-varying
capacitance to measure d-c high voltages. In its basic form, the
generating voltmeter comprises a motor-driven rotor that comprises
one or more metal vanes at ground potential. When the rotor is
rotated, it alternately covers and uncovers a stator that is
connected to ground through a load resistance. When the rotating
rotor and the stator are exposed to the electric field of a high
voltage electrode located a short distance away, an alternating
current flows through the stator and the load resistance to ground.
This current is usually rectified and filtered to produce a d-c
signal related in magnitude to the high voltage being measured.
The response time of such a basic generating voltmeter to voltage
changes is limited by the speed of the rotor vanes. Ordinary
generating voltmeters have a response time on the order of 100
milliseconds. This is much too slow to enable such a generating
voltmeter to be used for controlling high voltage d-c power
transmission systems. Precision resistors can be used for obtaining
a suitable voltage measurement in such power systems, but such
resistors are quite expensive and large.
SUMMARY
An object of our invention is to provide a generating voltmeter
that has a fast enough response to be used for controlling a high
voltage d-c power system.
Another object is to provide a rapid-response generating voltmeter
that can provide accurate polarity information as to the voltage
being measured.
Another problem that is present in the basic generating voltmeter
referred to hereinabove is that, because of the moving rotor, any
a-c frequencies which are present on the high voltage electrode
appear distorted at the output of the generating voltmeter. This
distortion makes it very difficult, if not impossible, to
accurately observe a-c transients using the basic generating
voltmeter.
Accordingly, another object of our invention is to provide a
generating voltmeter that is able to provide an output that
accurately shows a-c transients in the input to the generating
voltmeter.
In carrying out our invention in one form, we provide a generating
voltmeter for measuring a high d-c voltage between first and second
spaced-apart points. The voltmeter comprises a first electrode
operated at the potential of the first point and a second electrode
spaced from the first electrode and comprising a first stator
including a plurality of segments arranged in angularly-spaced
relation about a predetermined center. These first stator segments
are electrically interconnected so as to be at the same potential
as each other, and a first capacitor is connected between said
first stator and said second point.
A third electrode spaced from the first electrode and similar in
construction to said second eletrode has its angularly-spaced,
electrically-interconnected stator segments positioned physically
between the segments of the first stator, and a second capacitor is
connected between said second stator segments and said second
point.
The voltmeter further comprises capacitance-varying means for
periodically varying in opposite senses the capacitance between
said first electrode and said second electrode and the capacitance
between said first electrode and said third electrode. This
capacitance-varying means comprises a rotor having vanes normally
at the potential of said second point located between the first
electrode and the segments of the stators. Rotation of this rotor
causes the vanes to gradually cover the segments of one stator with
respect to said first electrode as said vanes gradually uncover the
segments of the other stator with respect to said first
electrode.
First circuit means senses the voltage across said first capacitor
while the rotor is rotating and develops a first intermediate
signal varying in magnitude directly in accordance with: the
instantaneous voltage then present across said first capacitor
minus a first error voltage substantially equal to the voltage
appearing across said first capacitor at the immediately-preceding
instant when the segments of the first stator were effectively
completely covered by the vanes of said rotating rotor.
Second circuit means senses the voltage across said second
capacitor and develops a second intermediate signal varying in
magnitude directly with the instantaneous voltage then present
across said second capacitor minus a second error voltage
substantially equal to the voltage appearing across said second
capacitor at the immediately-preceding instant when the segments of
said second stator were effectively completed covered.
Summing means develops an output substantially proportional to the
instantaneous sum of said first and second intermediate signals,
and this output is substantially proportional to the high d-c
voltage being measured. An important relationship contributing to
the accuracy of the voltmeter is that the sum of the capacitance
between said first electrode and said second electrode and the
capacitance between said first electrode and said third electrode
remains substantially constant during rotation of the rotor.
BRIEF DESCRIPTION OF DRAWINGS
For a better understanding of the invention, reference may be had
to the accompanying drawings, wherein:
FIG. 1 diagrammatically illustrates a basic generating voltmeter of
the prior art.
FIG. 2 is a sectional view through a generating voltmeter embodying
one form of our invention.
FIG. 3 is a reduced-size sectional view along the line 3--3 of FIG.
2, showing the segments of the two stators of the voltmeter.
FIG. 4 is a reduced-size sectional view along the line 4--4 of FIG.
2 showing the rotor of the voltmeter.
FIG. 5 is a schematic showing of our voltmeter showing its
electronic circuitry in diagrammatic form.
FIG. 6 depicts the equivalent circuit for the input components
associated with Stator I.
FIG. 7 shows certain waveforms associated with our voltmeter when
responding to a high voltage V such as depicted in curve (a) of
FIG. 7.
FIG. 8 shows certain waveforms associated with our voltmeter when
responding to a high voltage V such as depicted in curve (a) of
FIG. 8.
FIG. 9 shows certain waveforms associated with our voltmeter when
responding to a high voltage V such as depicted in curve (a) of
FIG. 9.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
COMPONENTS FOR DEVELOPING VOLTAGES e.sub.1 and e.sub.2
Referring first to FIG. 1, the basic generating voltmeter of the
prior art diagrammatically illustrated therein comprises a
stationary plate 10 placed in the electric field 12 of a high
voltage d-c electrode 14. The stationary plate is connected to
ground through a resistor 15. A movable plate or rotor 16 at ground
potential is located in the electric field between stationary plate
10 and the high voltage d-c electrode 14. If movable plate 16
remains stationary, no current will flow through stationary plate
10 to ground because the field at the stationary plate 10 is
constant in time. However, if the movable plate 16 is moved, the
field at the stationary plate will change, and this change in field
will cause a current to flow through resistor 15. When the manner
in which the moving plate passes over the stationary plate is
periodic, the mechanically-induced current can be related to the
d-c voltage level of the high voltage electrode 14. Thus, the basic
generating voltmeter can be used to measure the voltage level of
the high voltage d-c electrode.
There are several problems with generating voltmeters having the
type of construction shown in FIG. 1. First, they cannot determine
the polarity of the d-c voltage being measured. Second, a
significant d-c error is introduced if certain a-c frequencies are
superimposed on the d-c high voltage. Third, any a-c frequencies
which exist on the high voltage electrode will appear distorted at
the generating voltmeter output because of the moving plate, thus
making it difficult, if not impossible, to accurately observe a-c
transients with this device. These difficulties have been largely
eliminated in the rapid-response generating voltmeter described in
detail hereinafter.
Referring first to FIG. 2, our rapid-response generating voltmeter
comprises a high-voltage metal electrode, or plate, 20 that is
operated at the potential of a high voltage d-c line. It is desired
to continuously measure the voltage present between this plate 20
and ground. A grounded metal plate 22 substantially parallel to
plate 20 is located in spaced relation to the high voltage plate
20, and insulators 23 are used for supporting plate 20 on grounded
plate 22.
Grounded plate 22 contains a circular central opening 24, and
within this opening there are provided two electrodes in the form
of stators, each facing the high-voltage electrode, or plate, 20.
Each of these stators comprises a plurality of flat metal segments
disposed in a common horizontal plane and arranged in
angularly-spaced relationship about a central axis. These segments
are best shown in FIG. 3, where the segments of one stator (Stator
I) are designated 25, and the segments of the other stator (Stator
II) are designated 28. The individual segments of each stator are
electrically connected to each other, i.e., all of the segments 25
are electrically connected to each other; and all of the segments
28 are electrically connected to each other. As shown in FIG. 3,
the segments 25 of one stator are physically interposed between the
segments 28 of the other stator but are electrically insulated from
segments 28.
For supporting the stator segments 25 and 28, a stationary
insulating plate 30 is provided, as shown in FIG. 2. Insulating
plate 30 is mounted on the stationary housing 32 of an electric
motor 33. The motor housing is, in turn, mounted on a bracket 34 of
U-shaped cross-section that is a ground potential. Spaced metal
posts 35 support the grounded plate 22 on bracket 34.
Physically interposed between high voltage plate 20 on one hand and
the two stators on the other hand is a rotor 40 that is maintained
at ground potential. This rotor 40, which is best shown in FIG. 4,
is a flat disc of metal comprising four sector-shaped metal vanes
42 that are arranged in angularly-spaced relationship about the
center axis of the rotor. The vanes 42 are angularly spaced by an
amount slightly less than the angular spacing between the segments
of each stator. When the rotor 40 is rotated about its central
axis, the vanes alternately cover and uncover the stator segments
25 and 28. Each of the vanes 42 is of such a size that it can
completely cover any one of the stator segments 25 or 28 when
angularly aligned therewith. In a preferred form of the invention,
each of the vanes is slightly larger than each stator segment and
overlaps any angularly-aligned stator segment by a sufficient
amount that no significant electric field from the high voltage
plate 20 reaches the covered stator segment when angularly-aligned
with the vanes. When one of the vanes 42 is angularly aligned with
a segment of either stator, the other vanes are angularly aligned
with the remaining segments of the same stator. Thus, as the rotor
40 rotates, the vanes 42 act to cover the segments of one stator as
they uncover the segments of the other stator. When the segments of
one stator are completely covered by the vanes 42, the segments of
the other stator are at maximum exposure to the high voltage plate
20. Each of the stator segments 25 and 28 has substantially the
same area facing the high voltage plate 20.
An important feature of the rotor-stator relationship is that the
total area of the two stators that is exposed to the electric field
from the high voltage plate remains constant as the rotor rotates.
The significance of this feature will soon appear more clearly.
The rotor 40 is mounted on the shaft 45 of the electric motor 33.
This shaft is suitably electrically connected to ground. The motor
is a constant-speed motor which is energized whenever it is desired
to measure the voltage on high voltage plate 20. Normally, the
motor 33 will be continuously energized. In a specific embodiment
of the invention, the shaft speed of the motor is about 3585
rpm.
Referring to the schematic diagram of FIG. 5, it will be seen that
a capacitor C.sub.1 is connected between stator I and ground, and a
capacitor C.sub.2 is connected between stator II and ground.
Connected in parallel with capacitor C.sub.1 is a resistor R.sub.1
and in parallel with capacitor C.sub.2 is a resistor R.sub.2.
Typical values for these components are 0.05 microfarads for each
capacitor C.sub.1 and C.sub.2 and 100 megohms for each resistor
R.sub.1 and R.sub.2.
In FIG. 5, the capacitance between the high voltage electrode, or
plate, 20 and stator I is designated C.sub.1 ', and the capacitance
between high voltage electrode 20 and stator II is designated
C.sub.2 '. These capacitances C.sub.1 ' and C.sub.2 ' are very
small compared to the fixed capacitances C.sub.1 and C.sub.2,
preferably about 0.5 picofarads when the segments of each stator
are effectively completely uncovered by the vanes of the rotor.
When the segments of a given stator are effectively completely
covered by the rotor vanes, the capacitance C.sub.1 ' or C.sub.2 ',
as the case may be, is reduced to approximately zero. When the
rotor 40 is rotated at 593/4 revolutions per second, capacitances
C.sub.1 ' and C.sub.2 ' vary between zero and 0.5 picofarads at a
rate of 239 Hz. This rate follows from the fact that in the
illustrated embodiment there are four vanes on the rotor and four
segments on each stator, thus driving C.sub.1 ' and C.sub.2 ' to
zero four times each revolution.
An equivalent circuit for the input components associated with
stator I is shown in FIG. 6. It will be apparent from this figure
that the series combination of C.sub.1 ' and C.sub.1 acts as a
capacitive voltage divider. C.sub.1 ' is a time-varying
capacitance, and C.sub.1 is a fixed capacitance. Since C.sub.1 in
the specific embodiment described is 0.05 microfarads and C.sub.1 '
is 0.5 picofarads when the stator I is effectively fully uncovered,
the ratio of the divider under such conditions is 100,000 to 1.
Since R.sub.1 is assumed to be 100 megohms in this embodiment, the
decay time constant for the circuit R.sub.1 C.sub.1 is 5
seconds.
A corresponding equivalent circuit (not shown) can be drawn for the
input components associated with stator II.
Connected across R.sub.1 and R.sub.2 is electronic circuitry 50
that processes the signals e.sub.1 and e.sub.2 appearing across
R.sub.1 and R.sub.2, respectively, in a manner soon to be described
and produces an output voltage e.sub.o across output terminals 52
and 53 that is proportional to the voltage V between the high
voltage electrode 20 and ground. Each of the input voltages,
e.sub.1 and e.sub.2, to the electronic circuitry consists of a-c
and d-c components. The a-c component is formed by the product of
the rotor frequency and voltage V applied between the high voltage
electrode 20 and the ground. As will be apparent from FIG. 6, this
voltage, in the case of stator I, is capacitively divided between
the small time-varying capacitance C'.sub.1 and the large fixed
capacitance C.sub.1.
The d-c component of the input voltage e.sub.1 is produced by the
d-c voltage on the high voltage electrode 20. Referring to FIG. 6,
this d-c component cannot be faithfully maintained by the capacitor
divider because it is slowly drained away from capacitor C.sub.1 by
resistor R.sub.1, which protects the electronic circuitry.
Therefore, the d-c component of e.sub.1 (t) contains a d-c error
.epsilon..sub.1 (t) which increases as capacitor C.sub.1 is
discharged. This error in e.sub.1 (t) must be eliminated if the
high voltage is to be accurately reproduced by the generating
voltmeter.
The following is a step-by-step example of how the voltages e.sub.1
and e.sub.2 vary with time, with particular reference being had to
FIGS. 5 and 7.
First, assume that the rotor motor is off (so that the vanes are
not moving), and that the high voltage V has been off for an
extended time. Assume next that the grounded rotor vanes are
effectively completely covering stator II so that C.sub.2 ' is zero
and C.sub.1 ' is at its maximum value.
Now let us switch on the high voltage V at time t.sub.1, bringing
it rapidly up to +100 kV in a few microseconds (FIG. 7). The
voltage e.sub.1 will jump from zero to +1.0 volt because of the
voltage-dividing action of C.sub.1 ' and C.sub.1. Capacitor C.sub.1
immediately begins to discharge into resistor R.sub.1 with a
5-second time constant, causing e.sub.1 to decay slowly. The input
voltage, e.sub.2, from stator II remains at zero because C.sub.2 '
under the assumed conditions is zero.
Now assume that the rotor motor is switched on at time t.sub.2,
when e.sub.1 has decayed from +1.0 volt down to +0.9 volt. For this
example, we assume that the motor comes up to speed
instantaneously. As the rotor starts to cover the segments of
stator I, e.sub.1 starts to drop because C.sub.1 ' is decreasing.
Similarly e.sub.2 starts to rise as the rotor begins to expose the
segments of stator II to the field from the high voltage
electrode.
When the rotor has effectively completely covered the segments of
stator I at time t.sub.3, e.sub.1 will have decreased by 1 volt
from the +0.9 volts it had at time t.sub.2. Thus, at time t.sub.3,
e.sub.1 is at -0.1 volt. Note that the segments of stator I are
electrically shielded from the high voltage electrode at time
t.sub.3, but that the voltage e.sub.1 (t.sub.3) is not zero. This
non-zero voltage is the error voltage .epsilon..sub.1, which
erroneously indicates that the high voltage electrode is inducing a
voltage .epsilon..sub.1 on the segments of the stator I, even
though these stator segments are electrically shielded from the
high voltage electrode. As capacitor C.sub.1 discharges through
resistor R.sub.1, the error voltage .epsilon..sub.1 will slowly
increase until the average voltage of e.sub.1 is nearly zero, and
.epsilon..sub.1 = -0.5 V.
Referring to the Stator II voltage in FIG. 7, at time t.sub.3, the
segments of stator II are fully exposed to the field from the high
voltage electrode, so that e.sub.2 will be almost +1.0 volt. This
input voltage e.sub.2 will also slowly decay as C.sub.2 discharges
through R.sub.2. Note that e.sub.1 reaches a maximum when e.sub.2
is at a minimum and vice versa. Stated otherwise, e.sub.1 and
e.sub.2 are 180.degree. out of phase with each other.
ELECTRONIC CIRCUITRY 50
Electronic circuitry 50 is relied upon to process the
above-described input signals e.sub.1 and e.sub.2 and to derive
therefrom an output signal e.sub.o that is directly proportional to
the voltage V. This circuitry 50 comprises a preamplifier 60
connected across resistor R.sub.1 for amplifying the input signal
e.sub.1 to a more convenient voltage and current level. This
preamplifier is a conventional operational amplifier having high
input impedance, low input bias currents, and low d-c drift.
For deriving from the output of preamplifier 60 a signal that is
proportional to the error quantity .epsilon..sub.1 (t) depicted in
FIG. 7, we provide a sample-and-hold module, or circuit, SH-1. This
module is of a conventional design and acts when energized by a
suitable pulse to sample the instantaneous value of voltage e.sub.3
appearing at 62 and to provide an output voltage at 64 that is
equal to this instantaneous value of e.sub.3. This output signal
from module SH-1 remains constant until the module is energized by
another suitable pulse, at which time the same sampling-and-holding
operation is repeated until the module receives a new energizing
pulse.
Such an energizing pulse is applied to the sample-and-hold module
SH-1 each time the rotor 40 reaches a point where its vanes
effectively completely cover the segments of the stator I. This
instant is sensed by a suitable optical position sensor 70 that is
sensitive to the position of the rotor. Each time the leading edge
of a rotor vane intercepts a suitably positioned light beam, the
sensor 70 develops an output signal that is supplied to a pulse
generator 72, which immediately responds by supplying an energizing
pulse to the sample-and-hold module SH-1.
The sample-and-hold module SH-1 responds by sensing the
preamplifier's instantaneous value of output voltage e.sub.3 at 62
at this instant when the segments of stator I are effectively
completely covered and by developing an output signal at 64 of the
same magnitude and polarity as this instantaneous value of output
voltage e.sub.3. This output signal at 64 is maintained until the
segments of stator I are once again effectively completely covered.
The output signal at 64 is equal in magnitude to the error signal
.epsilon..sub.1 (t) in FIG. 7 times the gain of the preamplifier
60.
The output voltage e.sub.3 at 62 from the preamplifier and the
output voltage at 64 from the sample-and-hold module SH-1 are
supplied to a difference amplifier 71. This difference amplifier
subtracts the signal at 64, which in the example of FIG. 7 would be
a negative signal, from the signal e.sub.3 at 62 and produces an
output e.sub.5 proportional to this difference.
The effect of this subtraction is to restore the proper zero level
to e.sub.3. In this regard, consider the wave form of e.sub.3 (t),
which is the same as that of e.sub.1 (t), except for a scale
factor. If the d-c component e.sub.1 (t) were not drained by
resistor R.sub.1, as previously described, the value of e.sub.3
would be zero whenever the rotor vanes effectively covered the
segments of stator I. However, the voltage e.sub.3 contains an
error voltage .epsilon..sub.3 (t) which is caused by charge
drainage through R.sub.1. By subtracting this error voltage
.epsilon..sub.3 (t) from e.sub.3 (t), this error can be eliminated,
and the resultant signal will be zero whenever the segments of the
stator I are effectively covered. This subtraction takes place
inside difference amplifier 71, using the error signal from module
SH-1. Thus, the output signal at e.sub.5 (referred to herein as an
intermediate signal) has its zero level restored so that it
contains the correct d-c component.
Circuitry 50 contains an additional set of components for
processing a second input voltage (e.sub.2) to produce a second
intermediate signal (e.sub.6) in essentially the same manner as the
first input voltage (e.sub.1) is processed by the above-described
components to produce the first intermediate signal e.sub.5. More
specifically, a preamplifier 260 is provided for amplifying the
voltage e.sub.2, and this preamplifier corresponds to the
preamplifier 60. Preamplifier 260 has conventional means (not
shown) for adjusting its gain, and this gain is so adjusted that
e.sub.6 has an amplitude equal to that of e.sub.5 when the
voltmeter is measuring a constant d-c high voltage.
For deriving from the output (e.sub.4) of preamplifier 260 a signal
that is proportional to the error quantity .epsilon..sub.2 (t)
depicted in FIG. 7, a sample-and-hold circuit, or module, SH-2 is
provided. This module SH-2 is controlled by an optical position
indicator 270 and a pulse generator 272. Module SH-2 samples the
output voltage e.sub.4 from the preamplifier at each instant that
the vanes of the rotor 40 effectively completely cover the segments
of stator II and develops an output voltage at 264 that is held
until the segments of stator II are again effectively completely
covered. The output voltage e.sub.4 and that at 264 are subtracted
in a difference amplifier 271, and this difference is amplified by
amplifier 271 to provide an output in the form of intermediate
voltage e.sub.6 that is proportional to this difference. Difference
amplifier 271 has the same gain as difference amplifier 70. The
intermediate voltage e.sub.5 and e.sub.6, considered with respect
to their dominant frequencies, are out of phase with each other by
180.degree., as will be apparent from FIGS. 8(d) and 8(e).
For providing an output signal across the terminals 52 and 53 that
is of essentially the same wave form as the applied high voltage V,
the signals e.sub.5 and e.sub.6 are added together in a summing
circuit 80. The output from this circuit is a voltage e.sub.o
appearing across terminals 52 and 53. That this sum gives the
original, undistorted waveform shape can be understood by
considering the construction of the stator segments. As the rotor
40 turns, its vanes alternately cover and uncover portions of the
stator segments, and C.sub.1 ' and C.sub.2 ' each vary from 0 to
0.5 pf. However, the total area of the two stators which is exposed
to the high voltage remains constant, even though the rotor is
turning. Thus, the sum of capacitance C.sub.1 ' and C.sub.2 ' is
also constant, because this total capacitance depends on the total
area of the two stators which is exposed to the high voltage
electrode. Now, the voltages e.sub.5 and e.sub.6 each include the
modulating effect of the changing capacitances of C.sub.1 ' and
C.sub.2 ' so that neither correctly represents the high voltage
waveform. However, the sum signal (e.sub.5 + e.sub.6) is
proportional to the voltage which is seen by the constant
capacitance C.sub.1 ' + C.sub.2 '. This voltage is not modulated by
239 Hz, since the divider ratio C.sub.1 /(C.sub.1 ' + C.sub.2 ') is
constant. Therefore, the sum (e.sub.5 + e.sub.6) provides a correct
representation of the applied high voltage waveform for both a-c
and d-c components.
As pointed out hereinabeove, when the voltmeter is measuring a
constant d-c high voltage, e.sub.5 and e.sub.6 are of equal
amplitude. Since e.sub.5 and e.sub.6 are 180.degree. out of phase
with each other, they will sum to a constant value when the
voltmeter is measuring a constant value of d-c high voltage. It is
to be noted that e.sub.5 and e.sub.6 are 180.degree. out of phase
with each other in all their dominant and harmonic frequency wave
forms when representing a constant high voltage level.
In practice, the sample-and-hold modules SH produce narrow
switching transients, typically less than a microsecond wide and
about 100 millivolts high. These transients are reduced by an
output loss-pass filter (not shown) which is present in the summing
circuit 80. This filter is the primary limitation on the frequency
response of the voltmeter. In FIG. 8(f), these switching transients
are represented by the slight perturbations P in the e.sub.o wave
form. In the specific embodiment described hereinabove, these
perturbations have an amplitude less than about 0.1% of e.sub.o
after being filtered by the aforesaid low-pass output filter.
The flat frequency response of the rapid-response generating
voltmeter (which in one embodiment is .+-. 0.5% from d-c to 1 kHz)
is a consequence of the facts that very low applied frequencies are
shifted upward to the range of 239 Hz by the rotor motion, and that
the 239 Hz modulation is removed by summing e.sub.5 and e.sub.6.
All signals processed by the circuitry 50 are far away from zero
frequency so that divider impedances, offsets, and other
amplification problems do not introduce errors into the final
waveform. After all signal processing is completed, the 239 Hz
component is removed.
RESPONSE TO AN APPLIED STEP VOLTAGE
As an example of the combined a-c and d-c performance of the
rapid-response generating voltmeter, FIG. 8 is provided to show the
signals e.sub.1 to e.sub.o which would occur for an applied step
voltage. This applied voltage is similar to that of FIG. 7, except
for where the step occurs. At this point, the frequency response is
quite important. In one specific embodiment, the generating
voltmeter can respond to frequencies up to 1 kHz without
distortion, thus typically enabling the step to be accurately
reproduced within 0.5 msec. In FIG. 8 signals e.sub.5 and e.sub.6
(depicted in curves (d) and (e) show the effect of the SH modules,
and e.sub.o (depicted in curve f) shows the output waveform of the
electronic circuitry 50.
In FIG. 8(b) the stator I output voltage e.sub.1 following the step
shows the effect of the capacitor C.sub.1 discharging through
R.sub.1. The dotted-line error signal .epsilon..sub.1 (t), which is
negative, is subtracted from e.sub.1 and the difference amplified
to produce the wave form e.sub.5 of FIG. 8d. The similarly derived
corrected voltage e.sub.6 of FIG. 8e is added to that of FIG. 8d to
produce the wave form e.sub.o of FIG. 8f.
FIG. 9 illustrates how our voltmeter acts when the voltage being
measured is a negative polarity voltage V otherwise of the same
waveform as that of FIG. 8. The stator I output voltage e.sub.1
will have the waveshape shown in FIG. 9b. When the segments of
stator I are effectively fully uncovered, the instantaneous value
of e.sub.1 will be negative, and when the segments are effectively
fully covered, the instantaneous value of e.sub.1 will be positive.
The output voltage e.sub.3 from preamplifier 60 will have the same
waveform as e.sub.1. The sample-and-hold module SH-1 will thus
produce a positive polarity correction signal, which when
subtracted from the main signal e.sub.3 in difference amplifier 70
will produce a negatively-offset output at e.sub.5, as shown in
FIG. 9d. As shown in FIG. 9e, the circuit across stator II will
similarly produce a negatively-offset output at e.sub.6 when the
voltage V is negative. As shown in FIG. 9f, the two
negatively-offset outputs e.sub.5 and e.sub. 6 in the summing
circuit 80 will provide a negative polarity output signal at
e.sub.o, thus accurately reflecting the polarity, as well as the
magnitude, of the voltage V being measured.
GENERAL DISCUSSION
The following are some of the advantages of our rapid-response
generating voltmeter. It is very compact, especially when the
electrodes are contained in a pressurized dielectric gas such as
sulfur hexafluoride. Its response time is at least 1000 times
faster than the basic generating voltmeter referred to hereinabove
in the introduction and comparable with resistor-capacitor
dividers. Accuracies of .+-. 0.5% are attainable with out device,
which are as good as or better than with typical precision resistor
dividers. It is a very versatile device in that its mechanical and
electronic design can remain the same for all voltages. Only size,
spacing, and insulation of the high voltage electrodes need to be
redesigned to accommodate different voltages. It has a low power
consumption, consuming only about 100 watts as compared to a
kilowatt or more for comparable resistive dividers.
In terms of fidelity of output with respect to input, our
rapid-response generating voltmeter follows all variations up to a
high audio frequency in d-c or a-c high voltage, preserving
polarity information. This makes our device superior to the basic
generating voltmeter in any application where the voltage to be
measured is not "clean", i.e., wherever there are switching
transients, ripple, harmonic components, voltage fluctuations, and
the like.
While we have shown and described a particular embodiment of our
invention, it will be obvious to those skilled in the art that
various changes and modifications may be made without departing
from our invention in its broader aspects; and we, therefore,
intend herein to cover all such changes and modifications as fall
within the true spirit and scope of our invention.
* * * * *